Nearly half of the laminates manufactured for the printed circuit board industry use copper foil coated with an adhesive. These foils are laminated to a phenolic prepreg to make a low cost, copper clad paper phenolic laminate. The adhesives tend to fall into two categories: (1) a water dispersed adhesive composed of a copolymer of acrylonitrile and methacrylate and a phenolic resole, and (2) a solvent based adhesive composed of a phenolic resole, a polyvinylbutyral resin and a multifunctional epoxy, typically an epoxidized phenolic novolac.
Generally, these adhesives either provide good adhesion to metal and laminate as measured by peel strength, but less than satisfactory high temperature stability as measured in the solder blister resistance test; or they provide good high temperature stability, but less than satisfactory adhesion. This is because each of these goals are usually in direct conflict with one another. A material that is flexible enough to have high adhesion will generally exhibit too much flow at elevated temperatures to provide sufficient heat resistance. To have good heat resistance, an adhesive must be quite rigid and such rigidity reduces peel strength.
These problems have been overcome by the present invention. By virtue of the present invention there has been developed an epoxy adhesive composition that provides both good adhesion as measured by peel strength testing and good heat resistance as measured by solder blister testing.